The present invention relates generally to the molding of optical glass lenses and, more particularly, to methods for producing glass molds from yttria aluminosilicate glasses for molding optical glass elements therewith.
Various methods and apparatus for the compression molding of glass optical elements are known in the prior art. With these methods and apparatus, optical element preforms sometimes referred to as gobs are compression molded at high temperatures to form glass lens elements. The basic process and apparatus for molding glass elements is taught in a series of patents assigned to Eastman Kodak Company. Such patents are U.S. Pat. No. 3,833,347 to Engle et al., U.S. Pat. No. 4,139,677 to Blair et al., and U.S. Pat. No. 4,168,961 to Blair. These patents disclose a variety of suitable materials for construction of mold inserts used to form the optical surfaces in the molded optical glass elements. Those suitable materials for the construction of the mold inserts included glasslike or vitreous carbon, silicon carbide, silicon nitride, and a mixture of silicon carbide and carbon. In the practice of the process described in such patents, a glass preform or gob is inserted into a mold cavity with the mold being formed out of one of the above mentioned materials. The molds reside within a chamber in which is maintained a non-oxidizing atmosphere during the molding process. The preform is then heat softened by increasing the temperature of the mold to thereby bring the preform up to a viscosity ranging from 107-109 poise for the particular type of glass from which the preform has been made. Pressure is then applied to force the preform to conform to the shape of the mold cavity. The mold and preform are then allowed to cool below the glass transition temperature of the glass. The pressure on the mold is then relieved and the temperature is lowered further so that the finished molded lens can be removed from the mold.
U.S. Pat. Nos. 4,897,101 and 4,964,903, both to Carpenter et al., teach a method and apparatus for molding optical glass elements using molded glass molds. According to such method and apparatus, a metal master mold is first manufactured. Suitable materials for forming such master are Inconel 718, stainless steel type 420, tungsten carbide, and the like. The master surfaces may be coated or plated to minimize degradation of the molding surface through chemical attack, corrosion, denting, abrasion or adherence of the material being molded. Lens design data is apparently used to calculate the profile of the surface of the master. It is stated that the profile compensates for the different coefficient of thermal expansion of the lens and mold materials at the forming temperature to generate the required mold figure. Because Carpenter et al. teaches the use of metal master tooling, the process and method taught thereby is temperature limited. The glass molds formed with the metal tools must be made from glass having a relatively low molding temperatures (less than about 500xc2x0 C.). As a result, lenses molded using such glass molds must have a molding temperature that is even lower than that of the glass mold. The glass is taught as being acceptable for use with such glass molds had Tg no greater than 402xc2x0 C. Thus, the commercial viability of such method and apparatus of Carpenter et al. is, at best, questionable given the limited types of glass which may be used to mold glass optical elements using such method and apparatus. Indeed, there are very few commercially available glasses which can be used in the practice of such method and apparatus. As a result, apparently Carpenter et al found it necessary to develop new optical glass compositions that could be used with their method and apparatus. To date, only one commercially available optical glass, Coming C0550, meets the above requirements.
The use of glass molds has many benefits. Because the glass molds are replicated from a master mold of the inverse shape, the glass molds produced from the master will inherently have less dimensional variability. Glass molds are also more cost effective compared with diamond turned molds or ground and polished molds such as silicon carbide. A population of molds for multiple cavity manufacturing may be fabricated very quickly from glass molds compared with single-point turned molds or traditional ground and polished hard materials such as silicon carbide.
It is therefore an object of the present invention to provide a method for forming glass molding tools using an amorphous base material from which a working mold can be formed, wherein the working mold can be used to mold optical glass elements from glasses having a molding temperature in the range of from about 400xc2x0 C. to about 825xc2x0 C.
Another object of the present invention is to provide a method for forming glass molding tools from an amorphous material having a short or fragile viscosity curve characteristic.
It is a further object of the present invention to provide a method for making molded glass tools for use in molding optical glass elements wherein the base material used to mold the glass tool has a coefficient of thermal expansion in the range of from about 25xc3x97107/xc2x0 C. to about 70xc3x97107/xc2x0 C.
Yet another object of the present invention is to provide a method for making molded glass tools for use in molding optical glass elements wherein the coefficient of thermal expansion of the material of the mold tool preform may be altered from about 25xc3x9710xe2x88x927/xc2x0 C. to about 70xc3x9710xe2x88x927/xc2x0 C. without significantly altering the glass transition temperature or temperature for at which the glass material has a viscosity of at least about 1015 poise.
Briefly stated, these and numerous other features, objects and advantages of the present invention will become readily apparent upon a reading of the detailed description, claims and drawings set forth herein. These features, objects and advantages are accomplished by first making a master molding tool from a material having a maximum use temperature in the range of from about 900xc2x0 C. to about 2500xc2x0 C. The preferred material for this purpose is silicon carbide, produced by chemical vapor deposition. Other materials which can be used to fabricate the master mold tool include Vycor(copyright), fused silica, fused quartz, and various oxide, nitride, carbide, silicide, and boride ceramics and composites thereof The master tool is produced on computer-controlled grinding and polishing equipment to a very high degree of accuracy (peak to valley of 0.015 xcexcm). This master tool is, in turn, used in a glass molding process to produce working tools of opposite curvature. The working tools are then used to fabricate the finished optical glass lenses in a production mode. The method includes the calibration of curves and for a two-step molding process where the three materials (master tool, working tool, and lens) have different coefficients of thermal expansion and are used at two different temperatures (tool process and lens process). This is particularly important for aspheric surfaces. The system requires the identification of a hierarchy of glasses such that the working tool glass has a strain point above the process temperature for the lens glass. If the strain point of the working tool is not greater than the process temperature of the lens glass, curve creep will result as the working tool deforms a little with each molding cycle. An appropriate release agent is also required.
As stated above, once the master tool has been created, it is used to mold the working tool. Since the lenses ultimately to be molded through the use of the working tool have a molding temperature in the range of from about 400xc2x0 C. to about 825xc2x0 C., the working tool should have a Tg or strain point that is at least about 50xc2x0 C. greater than the molding temperature of the glass lenses. An amorphous-based oxide glass containing yttria, alumina, and silica is formulated to have a predetermined CTE. Such glass, called yttria aluminosilicate (YAS) glass, is very refractory with Tg""s ranging from about 880xc2x0 C. to about 910xc2x0 C. It has been surprisingly found that yttria aluminosilicate glasses have very short viscosity characteristics which allow for the fabrication of bubble-free melts for high-quality cast slabs. Furthermore, the composition of yttria aluminosilicate glasses may be adjusted to yield glasses with coefficients of thermal expansion that range from about 25xc3x9710xe2x88x927/xc2x0 C. to about 70xc3x9710xe2x88x927/xc2x0 C. without significantly altering the glass transition temperature of the glass. The term xe2x80x9csignificantlyxe2x80x9d as used herein with regard to altering glass transition temperature is intended to mean not more than about xc2x125xc2x0 C. Preferably, the composition of yttria aluminosilicate glasses may be adjusted to yield glasses with coefficients of thermal expansion that range from about 25xc3x9710xe2x88x927/xc2x0 C. to about 70xc3x9710xe2x88x927/xc2x0 C. with only about a xc2x110xc2x0 C. variation in Tg. Further, such YAS glass has a viscosity of less than about 102 poise at a temperature of less than about 1200xc2x0 C. A preform made of the YAS glass is molded to yield a working mold tool therefrom. This results in a working mold tool that includes a molded optical element forming surface, and that has a viscosity of at least about 1015 poise at the molding temperature the glass optics to be molded therewith. The YAS working mold tool is then used to mold glass optical elements such as lenses with the molded optical element forming surface. The CTE and geometry of the glass optical elements are used to determine a desired range for the predetermined CTE of the YAS glass. Other rare earth glasses may potentially be used in the practice of method of the present invention in place of yttria aluminosilicate glasses.
Preferably, the composition of rare earth and yttria aluminosilicate glasses may be adjusted to yield glasses with coefficients of thermal expansion that range from about 25xc3x9710xe2x88x927/xc2x0 C. to about 70xc3x9710xe2x88x927/xc2x0 C. also without significantly altering the temperature at which the rare earth and yttria aluminosilicate glasses have a viscosity of at least about 1015. Again, the term xe2x80x9csignificantlyxe2x80x9d as used herein with regard to altering the temperature at which the rare earth and yttria aluminosilicate glasses have a viscosity of at least about 1015 is intended to mean not more than about xc2x125xc2x0 C.
A first surface figure for the optical element to be molded with the working tool is defined. A second surface figure is computed for the master tool. A third surface figure for the working tool is then calculated based upon the first surface figure and the coefficient of thermal expansion of the optical element, the master tool, and the working tool, as well as the temperature at which the working tool is molded and the temperature at which the optical element or lens is to be molded. The master tool is ground and polished to achieve the second surface figure. A release coating is applied to the master tool and the master tool is used to mold the working tool.